A three-dimensionally stacked memory multilayered in the vertical direction and formed by collective processing has been proposed as a NAND flash memory.
In this three-dimensionally stacked memory, a cylindrical memory hole is formed through a plurality of electrodes stacked on a semiconductor substrate at once, and a memory layer is formed on the inner wall of the memory hole. After that, a semiconductor layer is formed inside the hole. Consequently, a memory string (NAND string) including a plurality of MONOS memory cells connected in series in the stacking direction can be formed at once.
The MONOS structure of the three-dimensionally stacked memory is formed by stacking insulating layers in order from the outside of the memory hole. In this case, a tunnel insulating film is an oxide film formed by not thermal oxidation but deposition. An oxide film formed by deposition has more defects than those of an oxide film formed by thermal oxidation. When a write/erase operation is repeated, therefore, electric charge is trapped in the tunnel insulating layer, and the charge retention characteristic deteriorates. This deterioration becomes significant when a large electric field is applied to the tunnel insulating layer.
In conventional multilevel write, a base threshold level at which write is started is a lower-end (most negative) level of multiple threshold levels. When writing data from this lower-end base threshold level to an upper-end threshold level, a large electric field is applied to the tunnel insulating layer. Consequently, electric charge trapping to the tunnel insulating layer is promoted, and the charge retention characteristic of the memory cell deteriorates.
As described above, in the three-dimensionally stacked memory in which the tunnel insulating layer must be formed by deposition, it is necessary to reduce the electric field to be applied to the tunnel insulating layer especially in a write/erase operation.